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From:
GearGrinder  Oct 1, 2006, 8:35 PM

Question:
About bladed spokes... and their real purpose
So I'm out for a windy, fall ride today on my Mavic Ksyrium's - complete with 24 bladed spokes. The wind is nuts, and ss I'm getting pushed around by the strong gusts, I get to thinking... are the wheels helping me or slowing me down?

THE BIG QUESTION: Why are the spokes bladed? Is it to look cool, or is is the same aero effect (but to a lesser degree) as a disc or deep-rimmed wheel? Obviously this is true in the case of a tri-spoke, but what about a wheel 20-24 regular bladed spokes? And if so, would more be better? e.g. would 40 bladed spokes have more of an aero effect?

Lot's of questions, but a 30mph headwind gives you a lot of time to think.

Josh:
The issue her isn’t so much about about whether spokes should be bladed or ovalized, but about the limitations of using aluminum for a spoke material. The reason the Ksyrium and similar wheels have such large side forces in a cross wind is that an aluminum spoke has to be about 2x the diameter of a steel spoke for similar strength, so to negate the very, very negative aero effects of this you really HAVE to blade the thing, round stainless steel spokes will cost you about 10-12 watts PER WHEEL, but round aluminum spokes will cost you nearly double that amount, creating a wheel that is slow beyond belief. Those of you who have ridden the old Spinergy Spox will remember, those wheels actually took nearly 30 watts more than a 32 spoke wheel at 30mph, and the same would be true of a round Ksyrium type spoke. But when the spoke is bladed, you have a lot of material you are moving around so you end up with a lot of side area.

A Ksyrium spoke is roughly 5mm x 1mm, compared to the CX-Ray we are using which is 2.2mm x 0.85mm. A standard round spoke is 2mm diameter, so whereas the aluminum spoke shows an increase in side area (and therefore side force) of roughly 250% the round spoke, the butted-bladed CX-Ray has an increase in side area of only 10%. There is good stuff about this on our website in the note about spoke shape posted earlier: http://www.zipp.com/...%20spoke%20shape.pdf

And actual drag and side force data on the Ksyrium shown in this note on wheel aerodynamics, you should note that the aluminum spoked Ksyrium shows a side force between the 202 and 303, where the 202 has a 3mm deeper rim and 2 additional spokes, and the 303 has a 16mm deeper rim and 2 additional (bladed spokes)...but both wheels have considerably better drag performance:

Rim Shape:
http://www.zipp.com/RimShape/tabid/103/Default.aspx